The mixing and combining of audio signals has been done in the context of multimedia computing. For example, U.S. Pat. No. 5,647,008 issued Jul. 8, 1997 to Farhangi et al. and assigned to Aztech Systems, Ltd. (hereinafter '008), discusses a method of digitally mixing multiple audio signals having independent sources, sampling rates and formats. Prior to mixing, the multiple audio input signals are converted to a common sampling rate before they are digitally mixed. To normalize the signals into a common sampling rate, the audio inputs are either interpolated or decimated at a rate that minimizes the loss of information from the audio input signals. Although some of these techniques can be utilized to enhance the present invention, the environment of digital audio radio (whether satellite or terrestrial based or both) has unique problems and configurations that require a different solution.
Satellite radio operators provide digital quality radio broadcast services covering the entire continental United States. These services offer approximately 100 channels, of which nearly 50 channels in a typical configuration that provide music with the remaining stations offering news, sports, talk and data channels. Digital radio may also be available in the near future from conventional analog radio broadcasters that will provide a terrestrial based system using signals co-located in the AM and FM bands.
Satellite radio has the ability to improve terrestrial radio's potential by offering a better audio quality, greater coverage and fewer commercials. Accordingly, in October of 1997, the Federal Communications Commission (FCC) granted two national satellite radio broadcast licenses. The FCC allocated 25 megahertz (MHZ) of the electromagnetic spectrum for satellite digital broadcasting, 12.5 MHz of which are currently owned by one company and another 12.5 MHz of which are owned by the assignee of the present application “XM Satellite Radio Inc.”
The system plan for each licensee presently includes transmission of substantially the same program content from two or more geosynchronous or geostationary satellites to both mobile and fixed receivers on the ground. In urban canyons and other high population density areas with limited line-of-sight (LOS) satellite coverage, terrestrial repeaters will broadcast the same program content in order to improve coverage reliability. Some mobile receivers will be capable of simultaneously receiving signals from two satellites and one terrestrial repeater for combined spatial, frequency and time diversity, which provides significant mitigation of multi-path interference and addresses reception issues associated with blockage of the satellite signals.
In accordance with XM Satellite Radio's unique scheme, the 12.5 MHZ band will be split into 6 slots. Four slots will be used for satellite transmission. The remaining two slots will be used for terrestrial reinforcement.
In accordance with the XM frequency plan, each of two geostationary satellites will transmit identical or at least similar program content. The signals transmitted with QPSK modulation from each satellite (hereinafter satellite I and satellite 2. For reliable reception, the LOS signals transmitted from satellite 1 are received, reformatted to Multi-Carrier Modulation (MCM) and rebroadcast by terrestrial repeaters. The assigned 12.5 MHZ bandwidth (hereinafter the “XM” band) is partitioned into two equal ensembles or program groups A and B. Each ensemble will be transmitted by each satellite on a separate radio frequency (RF) carrier. Each RF carrier supports up to 50 channels of music or data in Time Division Multiplex (TDM) format. Currently, there is no existing receiver that can combine the various channels from the TDM format into a combined audio signal.
Thus, in a digital audio radio system such as the system described above, a need exists for a device that enables a receiver to combine audio from various channels from a signal transmitted in the TDM format.